Optical technology offers the capability of transmitting a plurality of signals on a single fiber. As the use of optical technology increases, the number of signals transmitted on a signal fiber also increases. Currently, signals of four, eight, or sixteen different wavelengths are transmitted over a single fiber.
In order to transmit a large number of signals on a given cable, signals carried on a plurality wavelengths are multiplexed together for transmission. After the multiplexed, multi-channel signal reaches the destination, the signals are demultiplexed to access the information carried by a single channel. Conventional wavelength division multiplexers ("WDMs") are used to combine individual signals as well as separate a composite signal into individual wavelength signals.
To separate a composite signal into its components, some conventional wavelength division multiplexers utilize a plurality filters cascaded in series to isolate each channel. Each filter passes light centered around a single wavelength. A filter is typically provided for each of the component wavelength. The output of each filter is the signal centered around a single wavelength. As each component wavelength is separated from the composite signal, the remaining portion of the composite signal is passed on to the next filter in the series. Thus, the signal is separated into its components.
Typically, the composite signal is input to a conventional WDM via an optical fiber. The fiber is typically held by a capillary. The composite signal is then provided to a graduated index of refraction (GRIN) lens for collimating. The signal is then transmitted to the filter. The filter transmits light centered around a particular wavelength. This wavelengths corresponds to one or more channels. The remainder of the composite signal is reflected back to another GRIN lens, capillary, and optical fiber combination. The remainder of the signal may then be transmitted through the optical fiber to a second pair of capillaries, a second pair of GRIN lenses, and filter combination. The second filter transmits light for a second channel. This process continues until the signal is broken into its composite wavelengths.
In one type of conventional wavelength division multiplexer, the filter is fixed at the end of the GRIN lens. Consequently, the angle between the filter and the incident light is fixed. This angle is approximately two degrees. Although such a conventional wavelength division multiplexer can separate a composite signal into its components, the pass band of the filter is fixed. The pass band is the range of wavelengths transmitted by the filter. Consequently, the filter for each channel must be manufactured to have the correct pass band for that channel. The filter itself will, therefore, be subject to tighter specifications in order to ensure that the appropriate portion of the signal is transmitted. The tighter specifications make the filter more expensive. In addition, there may be cross-talk between the incoming composite signal and the portion of the composite signal that is not transmitted by the filter.
In another conventional wavelength division multiplexer, the filter is not affixed at the end of the GRIN lens. Thus, the angle between the GRIN lens and the filter can be adjusted in order to tune the pass band of the filter so that the filter transmits light of the appropriate wavelength. As this angle is adjusted, the angle of incidence between the direction of propagation of the composite signal and a direction approximately perpendicular to the surface of the filter is changed. Changing the angle of incidence changes a central wavelength. The central wavelength is the wavelength around which light transmitted by the filter is centered. Consequently, the filter can be tuned so that the central wavelength is the appropriate wavelength for a desired channel.
Although the filter can be tuned by adjusting the angle between the filter and the GRIN lens, the angle used is typically relatively large. The band with of the filter's pass band decreases at higher angles. The insertion loss for the filter is also increased. In addition, light polarized in the S direction is transmitted differently from light polarized in the P directions. As a result, the polarization dependent loss is dramatically increased.
Accordingly, what is needed is a system and method for tuning a filter, particularly in a wavelength division multiplexer, which does not result in high insertion loss and a high polarization dependent loss. The present invention addresses such a need.